Support CONFIG_SITE being a list of entries.

Support CONFIG_SITE being a list of entries.

Instead of treating CONFIG_SITE as a single path, treat it as a
space-separated list of paths and load them in order.

Also remove the special-casing of entries starting with a dash, this is
redundant as they'll be caught by the wildcard case.

Finally add a test case to verify that multiple files are loaded
correctly.

* lib/autoconf/general.m4 (AC_SITE_LOAD): Treat CONFIG_SITE as a
  space-separated list of scripts to be sourced.  Simplify handling
  of default config.site locations using this capability.
* tests/base.at (AC_CACHE_CHECK): Test loading of multiple site files.
* doc/autoconf.texi (Site Defaults): Update documentation of CONFIG_SITE.